There are a wide variety of recording media for recording and saving digital audios, images, moving images and document files and data files generated by computers. This variety includes optical discs. Above all, DVDs (Digital Versatile Disks) have higher densities and higher capacities than conventional CDs (Compact Discs) and have spread as media replacing presently prevailing VTR (Video Tap Recorders) also in the field of recording apparatuses. Further, in recent years, researches on next-generation optical discs with higher recording densities using blue-violet semiconductor lasers have been made in laboratories. Early appearance and prevalence are waited for. An optical pickup is necessary to record or reproduce data on or from these optical discs.
An optical pickup for recording or reproducing information on or from an optical disc generally includes a light source, an objective lens for condensing a beam emitted from the light source on the optical disc, and a detector for detecting the beam reflected by the optical disc. Since a semiconductor laser as the light source emits a beam from an end surface of its thin active layer, the beam has an elliptical shape and a ratio of a minor axis to a major axis of this elliptical shape is about 1:3. Upon recording information on the optical disc, it is desirable to shape the elliptical beam into a circular beam in the light of improving light utilization efficiency.
An anamorphic prism or an anamorphic lens is generally used as such an element for shaping the beam. Since the anamorphic prism needs to be used in a parallel beam, it hinders the miniaturization of the optical pickup. Accordingly, by using the anamorphic lens capable of shaping a divergent beam, a small-size optical pickup with high light utilization efficiency can be realized.
Other elements actually constituting the optical pickup include a reflective optical element and an objective lens. These elements are generally likely to have astigmatism as an initial property. A beam shaping element formed of an anamorphic lens has a property of giving astigmatism due to the deviation of a distance from an emission point from a designed value. The optical pickup can conversely correct the astigmatisms of the reflective optical element and the objective lens utilizing this property. By having such a construction, a spot formed on the optical disc by a beam emitted from the objective lens of the optical pickup can have a better quality.
FIG. 21 show one prior art of a retaining structure for a beam shaping element. This prior art is disclosed, for example, in patent literature 1 below. In this prior art, an occurrence of astigmatism resulting from a refractive index change can be suppressed by equalizing a distance “s” from an emission point to a first surface S1 of a beam shaping element L and a thickness “t” of the beam shaping element L, and an occurrence of astigmatism resulting from an environmental temperature change can be suppressed by equalizing a linear expansion coefficient αL of the beam shaping element L and a linear expansion coefficient αH of a retainer H.
Patent Literature 1:
Japanese Unexamined Patent Publication No. 2003-178480
However, the construction shown in the prior art considers the astigmatisms that occur due to a distance change between the beam shaping element and the emission point by an environmental temperature change and the like. Thus, it is only designed to reduce astigmatism adjustable sensitivity between the emission point and the beam shaping element. Further, a LD (Laser Diode) as a light source element having the emission point is generally covered by metal and the like. Specifically, if the beam shaping element designed as shown in the prior art is used, a moving distance increases due to a low adjustable sensitivity in the case where the distance between the emission point and the beam shaping element is changed to correct the astigmatisms of the reflective optical element and the objective lens. Thus, there are cases where the beam shaping element and the cover of the LD touch each other, which presents a problem that the astigmatism of the spot formed on the optical disc by the beam emitted from the objective lens cannot be perfectly corrected.
In the prior art, only the retaining structure for the light source element and the beam shaping element is described, but a method for actually fixing the beam shaping element is not described. Even if the construction shown in the prior art is adopted, there is a problem that astigmatisms higher than those calculated in relation to a temperature change occur depending on a fixed position by an adhesive if the adhesive is, for example, used to fix the beam shaping element.